Gametogenesis
Embryology course 2010
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Arrangement of the male gonads
Arrangement of the female gonads
Spermatogenesis
Oogenesis
Spermatogenesis
Structure of the germinal
epithelium
Developmental stages of
spermatogenesis
• A-spermatogonium
• B-spermatogonium
• Primary spermatocyte (= spermatocyte
order I)
• Secondary spermatocyte (= spermatocyte
order II)
• Spermatid
• Sperm cell (= spermatozoon)
The temporal course of
spermatogenesis
Mitosis of the
spermatogonia
16 days
Up to the primary
spermatocytes
First meiosis
24 days
For the division of the
primary spermatocytes to
form secondary
spermatocytes
Second meiosis
A few hours
For engendering the
spermatids
Spermiogenesis
24 days
Up to the completed
sperm cells
Total
~64 days
Spermatocytogenesis
• spermatogonia:
• homonymous division
• heteronymous division
• primary spermatocytes (I): enter into the
first meiosis
• secondary spermatocytes go directly
into the second meiosis
• spermatids
Local course of
spermatogenesis - the
spermatogenesis wave
Structure of the sperm cell
• Nuclear condensation: thickening and
reduction of the nuclear size, condensation of
the nuclear contents into the smallest space.
• Acrosome formation: Forming a cap
(acrosome) containing enzymes that play an
important role in the penetration through the
pellucid zone of the oocyte.
• Flagellum formation: generation of the sperm
cell tail.
• Cytoplasma reduction: elimination of all
unnecessary cytoplasm.
• The neck contains the two centrioles (proximal
and distal) among other things.
• The mid piece consists of a sheath of ringshaped mitochondria grouped around the
axoneme to provide the energy for the flagellar
movement.
• The principle piece has a sheath of ring fibers
around the axoneme.
• The tail consists of only the 9+2 structure of the
axoneme
Leydig's interstitial cells and
hormonal regulation
Oogenesis
Group
type of
egg
type of
cleavage
Amphioxus,
mammals
isolecithal,
Oligolecitha
or alecithal
holoblastic (or
complete):
initial
cleavage
planes
extend
through
egg)
amphibians,
lampreys
mesolecithal
holoblastic
reptiles
telolecithal
meroblastic
(incomplete)
: initial
cleavages
planes
limited to
animal pole,
the region
blastula
blastula cavity
sphere with
single layer
wall
large, central
sphere
sphere with
layered wall
small sphere
cell disk on
surface of
yolk,
bilaminar
flat space
between
epiblast and
hypoblas
developmental sequence
• Primordial germ cell - oogonium primary oocyte - primary oocyte in the
dictyotene
• Birth
The continuation of the development /
maturation of the oocyte begins again
only a few days before ovulation
• The developmental sequence of a follicle
goes through various follicle stages:
• Primordial follicle - primary follicle secondary follicle - tertiary follicle
(graafian follicle)
• Since a follicle can die at any moment in
its development (= atresia), not all reach
the tertiary follicle stage.
The follicle stages from primordial
follicle to tertiary follicle
Temporal course of the number
of germ cells / follicles
• Phase A:
Primordial germ cells grow, proliferate 6th to 8th week.
Phase B:
Spurt of growth: cellular clones of the oogonia are formed, whereby
the cells remain connected with each other through cellular bridges;
9th to the 22nd week.
Phase C:
The oogonia become primary oocytes that enter the prophase of the
first meiosis; 12th to the 25th week.
Phase D:
The primary oocytes become arrested in the prophase: the primordial
follicles are created; 16th to the 29th week.
Phase E:
At around the 14th week a quantitatively increased decline in the
number of germ cells commences as well as atresia in all of the
follicle stages.
Spermatogenesis
Oogenesis
Number of gametes
Principle: continuous production. Although
from puberty to old age sperm cells are
constantly being engendered, the
production is subject to extreme
fluctuations regarding both quantity and
quality.
Principle: Using up the oocytes generated
before birth.
Continual decrease of the oocytes,
beginning with the fetal period.
Exhaustion of the supply at
menopause.
Meiotic output
Four functioning, small (head 4 mm), motile One large, immotile oocyte (diameter 120
spermatozoids at the end of the meiosis
mm) and three shriveled polar bodies
are left at the end of the meiosis
Fetal period
No meiotic divisions
Entering into meiosis (arrested in the
dictyotene stage)
No germ cell production
Production of the entire supply of germ
cells
The ovarian cycle
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the ovarian cycle (follicle maturation)
that peaks in the ovulation and the
subsequent luteinization of the granulose
cells
cyclic alterations of the endometrium
that prepare the uterine mucosa so
fertilized oocytes can "nest" there. In the
absence of implantation, the mucosa will
be eliminated (menstrual bleeding)
• The control circuit of the hormonal cycle
has two essential control elements:
– The pulsatile liberation of GnRH, as well as
FSH and LH
– The long-loop feedback-effect of estrogen
and progesterone on the hypothalamichypophysial-system (these two hormones are
synthesized in the [ready to rupture] follicle and so originate in
the ovary, thus the name "long loop").
• Follicle phase: recruitment of a so-called
follicle cohort and, within this, the selection
of the mature follicle. This phase ends
with ovulation. Estradiol is the steering
hormone. Normally, it lasts 14 days, but
this can vary considerably!
• Luteal phase: progesteron production
by the "yellow body" (= corpus luteum) and
lasts 14 days (relatively constant).